1. Field of the Invention
This invention relates to a process and apparatus for the production of carboxylic acid esters.
2. Description of the Related Art
Esterification is a well known equilibrium limited reaction involving reaction of a mono-, di- or polycarboxylic acid (or, in suitable cases, an acid anhydride) with an alcohol or phenol component. Such an alcohol or phenol component can be mono, di- or polyhydric.
In the formation of a monoester, for example, the reaction can be summarised: EQU R.sup.1 COOH+R.sup.2 OH.revreaction.R.sup.1 COOR.sup.2 +H.sub.2 O,(1),
where R.sup.1 is a hydrogen atom or a monovalent organic radical and R.sup.2 is a monovalent organic radical. When an acid anhydride is used as the acid component, the reaction occurs in two stages: ##STR1## where R.sup.3 is a divalent organic radical and R.sup.4 is a monovalent organic radical.
Although the reaction of equation (2) above will normally proceed in the absence of added catalyst and although the organic acid may autocatalyse reactions (1) and (3) to some extent, it is normal practice to add a catalyst to the esterification reaction mixture in order to accelerate the reaction.
Perhaps the most widely used catalysts are sulphuric acid and organic sulphonic acids, such as p-toluenesulphonic acid. Although these catalysts are efficient, they are homogeneous catalysts and a neutralisation step is necessary before ester purification can be attempted. Typically washing with an alkali, such as sodium hydroxide solution, is used in such a neutralisation step. As esterification is an equilibrium process, a disadvantage of this procedure is that the washing step also results in removal of any unreacted carboxylic acid component in the wash liquor. Normally it is uneconomic to attempt to recover the unreacted acid from its salt in the wash liquor so that this may represent a significant loss of process efficiency. In addition some ester may be lost in this washing step. The losses of ester in the aqueous alkali phase will depend on the solubility of the ester in such solutions. The sodium salt of a partially esterified polycarboxylic acid will also be soluble in the aqueous alkali liquor. Loss of such partially esterified polycarboxylic acid will, in this case, represent a loss both of acid moieties and also of alcohol moieties. Furthermore the disposal of the wash liquor may present environmental problems which may be aggravated by the presence of the organic carboxylic acid salt in the wash liquor. In addition, particularly when long chain fatty acids are involved, problems may arise in the washing step due to formation of emulsions that are stabilised by the alkali metal fatty acid salts, which are surface active, and that are often difficult to separate into their component aqueous and organic phases. The stability of such emulsions is known to vary in an erratic way, thus making the design of, the organic phase/aqueous phase separation equipment difficult. Therefore it is difficult to practise an esterification process with a homogeneous catalyst on a continuous basis. As a result batch processing is usually adopted, a factor which may affect product quality from batch to batch. An additional disadvantage of the use of such homogeneous catalysts is the risk of contamination of the ester with sulphur-containing components. Such sulphur-containing components can interfere seriously with subsequent chemical processing steps.
Recently there has been proposed, as a result of work carried out in our laboratories, a continuous process for the production of dialkyl maleates which utilises an acidic ion exchange resin as catalyst. This proposal is described in EP-A-0255399 and in WO-A-88/00937. According to this proposal a dialkyl maleate is produced by flowing a liquid feed mixture containing monoalkyl maleate in countercurrent to a flow of vaporous alkanol vapour, so that the liquid phase encounters progressively drier alkanol vapour in passage through the catalyst-containing esterification zone or zones. In one embodiment the resin catalyst is wrapped in mesh packages and packed as a trickle bed in a tower down which the liquid phase flows against an upcoming alcohol vapour. In another embodiment a plurality of continuously stirred tank reactors is used with the liquid phase passing from one reactor to the next succeeding reactor of the series whilst the alcohol vapour flows from each reactor to the preceding reactor of the series.
The proposals described in EP-A-0255399 and in WO-A-88/00937 are somewhat complex. The tower concept requires that the ion exchange resin be wrapped in individual mesh packages. The multi-reactor system requires continuous operation of stirrer motors and is somewhat difficult in practice to control, besides requiring a significant site area for erection.
A form of gas-liquid contact column is described in U.S. Pat. No. 3,394,927. DE-B-1009749 describes a column reactor for hydrogenating unsaturated oils. In FR-A-1384683 there is described a multi-compartment column reactor in which catalytic reactions can be carried out between a gas and a liquid flowing in countercurrent from compartment to compartment and in co-current within each compartment.